This invention relates to a three-phase brushless dc motor using two position detecting elements to obtain the signal necessary for sequentially energizing the motor and being capable of obtaining one PG reference pulse signal for every rotation of the motor, said PG pulse being used for phase detection of the motor. That is, the PG reference pulse indicates when a predetermined reference point on the motor passes a fixed point.
Conventionally, a brushless dc motor with three-phase exciting windings usually has three position detecting elements, disposed at intervals of an electric angle of 2.pi./3, which are arranged to detect a three-phase position signal corresponding to the rotating position of the rotor to thereby cause currents to flow sequentially in the three-phase exciting windings in the width of the electric angle of 2.pi./3.
Such a three-phase brushless dc motor needs the three position detecting elements which are required to be disposed at regular angular phase differences with a high mechanical accuracy, a small-sized motor requiring a particularly high accuracy, thereby creating a problem of a high manufacturing cost. For example, in the case where the three-phase brushless dc motor directly drives the rotary magnetic heads of a video tape recorder, a signal indicating the rotary phase of the rotary magnetic head is required in view of its function to thereby need a reference pulse signal output (to be hereinafter called the PG pulse) for every rotation of the motor. A simple method to obtain the PG pulse signal is to use the two-pole-magnetized permanent magnet which is also used for the motor, the magnetic flux of the permanent magnet being detected by sensitive elements. Such a motor, however, does not have a large flywheel effect due to the low ripple frequency of the torque generated by the motor when a small number of magnetic poles are used. Hence, a three-phase brushless dc motor with a rotary magnet of four or more poles is generally adopted, and which is provided with a small magnetic piece located on its rotary portion to thereby detect the magnetic flux, thus obtaining an accurate rotation of the motor and a PG reference pulse signal. This method, however, is defective in that the mounting of the magnetic piece on the rotary portion deteriorates its dynamic balance, and increases the number of parts and the manufacturing processes so as to thereby cause a high manufacturing cost.